My review here is going to focus mostly on sound quality (and ANC). The tech features - bluetooth, mobile app, battery life, etc. - that all works as advertised, and there isn't much more to add.I think one of the most important things to call out when discussing sound is perspective. I would call myself a "basshead audiophile". By this I mean, I enjoy focused listening and enjoying the details in music - BUT for me, this still includes bass. This is likely tied to the genres of music I listen to most, which are EDM, Hip-Hop, R&B, Alternative, Pop, some Rock. I own eight pairs of headphones right now for different purposes, and the M3s sit in what I would call the "all arounder" category - meaning they are versatile enough to enjoy any of the genres I listen to when I want to have a session of focused listening. What makes them great is that they also have the features to be a "daily driver" as well - meaning, the headphones you use in most situations overall. For the price, in my view, they are a steal. Ok, let's get into the sound, which will focus on a few categories - loudness/clarity, tonal balance, bass, mids, highs, soundstage/positioning, ANC, listening mode, other considerations.LOUDNESS/CLARITY:These headphones have relatively big dynamic drivers, and they produce big sound. Optimal listening volume for me has been around the 70-90% range, depending on how I'm feeling. These can be connected via bluetooth, USB-C or 3.5mm, and I've found USB-C gets slightly louder than bluetooth (though both are more than loud enough IMO), and of course the loudness when connected to 3.5mm will depend on the power of the device to which you're connecting the cans.Clarity is very good as well. Straight out of the box, cranked at max volume, distortion is not an issue. Since you can EQ these with the Sennhsiser app, it is possible to get them to distort by cranking up all of the EQ settings and volume to max, but that is a pretty extreme scenario.TONAL BALANCE:Out of the box, these are definitely a "warm" sound signature - meaning, the low end of the spectrum is elevated. For the M3s, this starts at the lower mids and continues to the sub-bass. This does NOT mean the mids and highs are overshadowed or not present; that is not the case. I would call the balance low-end forward, but that low end still layers into the rest of the spectrum very well. Benefit of that Sennheiser engineering. Keep in mind the smart control app allows you to adjust the tonal balance via EQ, and the headphones respond very well to EQ adjustments (and allow you to save multiple EQ profiles for quick swaps).An example here, I listen to a lot of Trance EDM that has very important low end but at also has a lot of high-end details. My most-used EQ ups the lows by 1dB and ups the highs by 3dB. They sound great out of the box, but being able to tweak to your liking is even better. Very versatile.BASS:These cans are a bass lover's dream. The bass volume and impact is there without question. Out of the box there is already significant bass volume, and it can be EQ'd to be straight up banging - to the point that your ears will vibrate. It's a woofer on your head situation. That said, the bass is not at all messy or overly boomy unless the track calls for it. The set always feels in control and cleanly layered into the mix. The bass has good depth at the mid-bass and sub-bass levels. The cans also shine on bass detail. The genres I listen to have a lot of tracks that have fast-moving, layered bass lines, and they really shine on this set.MIDS:The two things that immediately come to mind when I think about the mids on this set are recessed and smooth. The way the mids are presented in the overall soundstage (will get to that shortly) makes them feel slightly recessed, but not in a bad way. Vocals, for example, come through clearly but always in their own distinct space, and that space is very rarely "right in your face". If you've ever listened to any of the AirPods lines, they are very vocal-forward and bright; that is not the case here (unless you EQ them up). The smoothness and clarity of the mids plus the combination of the natural warmth of the set really work well together.HIGHS:The highs on these are also very good but not necessarily the star. By that I mean the lows and mids are an A, and the highs are maybe an A- or B+. This is mostly because the highs are not as sharply elevated as the lows, so there is some roll-off at the very upper ends. This can be corrected with EQ (as I mentioned above), and the results are good, but I have to be fair. What I enjoy about the highs on this set is that they are not fatiguing or overly bright - meaning, you don't get any of that intolerable high-end reverb with symbols or ambient noises. This means you can crank the volume to max and get great clarity without annoying your eardrums.SOUNDSTAGE + POSITIONING:This is another star of the show you get by dealing with Sennheiser products. The soundstage on these is VERY good and what takes them to the next level for me. I would describe it as "intimate but open." Overall, the soundstage presents a very close/intimate listening experience. Sounds appear either insider your head (often with vocals) or very close to you. The width is very good, as you'll often get sounds that appear just at the edge of your periphery. The height is also impressive, and I have more than once found myself able to separate sounds that appeared to becoming from above or below in tracks. What impresses me most about the soundstage here, however, is the fullness. Many over-ear headphones (looking at you AirPods Max), have a wide soundstage, but they are compressed - sounds all obviously originate from the far left, far right, or center. The Sennheisers do an outstanding job of blending these together. Sound truly does appear to come from all around. The combination of the soundstage and tonal clarity really impresses. I am a heavy Apple Music listener, and with these headphones, I will not use the Dolby Atmos format because the artificial soundstage produced by that actually sounds worse than the natural soundstage of these headphones (more on that below). Soundstage is a solid A, especially for the price.ANC:Again, have to be fair, the ANC here isn't great. I'd give it a B-/C+ purely because there are other options out there that are far better - Bose and AirPods Max come to mind. These do completely cover your ear, and the pads provide decent isolation without any ANC turned on. With the ANC turned on, it will block out some passive noise that I would say is good enough in most environments, as long as you have music playing, even at a low volume. If your goal is to sit on an airplane/train with headphones on in total silence with no music playing, these won't give you the best result. This is a sound quality-first headphone, and the relative weakness of the ANC showcases that. The positive about the weaker ANC is that it has a very minimal effect on the sound quality. There is a slight difference that's most noticeable in the low-end resonance, but overall, not significantly different. I tend to leave ANC off until I need it. The strength of the ANC is also adjustable in the app for those that have sensitivities to the affects of ANC, and there is a decent transparency mode, so you can ear outside noise when you have the set on.LISTENING MODES:As mentioned earlier, the M3s allow you to connect to your source in three ways: Bluetooth, USB-C and 3.5mm. In short - they are all at least very good.Bluetooth mode is likely the way most will listen, and everything I have said about the sound quality above absolutely applies in bluetooth mode. These were designed to be wireless cans, and they have great bluetooth performance. That said, if you're still reading this review that means you likely know enough to know that wired is almost better than wireless, and that is true here as well. You can use these headphones as a direct USB-C to USB-C set of headphones - for example through an iPad Pro, iPad mini or a phone that has a USB-C port. This has two advantages - #1 it allows you to listen to lossless music, as this is a purely digital transmission method that supports lossless files, and #2 it allows you to avoid the interference of wireless transmission. Listening to music in this way takes everything I said about the sound and elevates it another 1/2 letter grade. It's like going from a regular 4K tv to a 4k TV with Dolby Vision HDR. It's just a smidge clearer and more defined - and noticeably so. Also, in this mode, the device is still powered (and can charge while being used), so you still get the benefit of all of the digital features - ANC, EQ, on-set controls, etc.You can also drive these totally unpowered by plugging in an AUX 3.5mm cable directly to a source. First, I have to knock Sennheiser for including a pretty crappy aux cable for this with a weird screw-in 2.5mm end that goes into the headset itself. Odd choice that while may have a purpose, makes replacing what is a pretty crappy cable fairly difficult. The sound results you will get from this method depend largely on what you are plugging the headphones into because in this method, the headphones are off, so you are driving the headphones with your own DAC and Amp. I personally tried this with a Dragonfly Cobalt and did not get great results, but I have seen many across the net that have had good results with this connection type. I will say, this is really not the way this set was meant to be used, and if you really just want a pair of non-powered headphones to plug into a 3.5mm jack, there are better options out there from Sennheiser and others that are cheaper.OTHER CONSIDERATIONS:A few other things I want to mention as they relate to sound quality.The first is comfort. I find these headphones extremely comfortable. The cups sit snugly over the ears, but the cushions are soft enough that they are not painful. They also do not get hot. However, I have noticed two issues. They do not perform well when I am wearing glasses. The pads don't form well around the arm of the glasses, so you get obvious sound drop/off leakage. Also, depending on how they fit against your ears, if you move your head a lot while listening, it may affect the sound a bit. This is not an awful thing and is very common with over-ear headphones, but these headphones don't have any tech in them that adjusts the frequency based on the fit over your ear or objects in the way (glasses). There are benefits and costs to doing this. The benefit of doing it, like the AirPods max, is even with glasses on or moving your head a lot, the AirPods max readjust the tonal balance based on the fit at the millisecond level to minimize the impact. The trade-off is the sound is artificially modified in a way that the user does not control - and this is likely against the ethos of Sennheiser. Again, this is not a huge issue, but worth mentioning.The other thing worth mentioning is use for other things like TV, podcasts/books and gaming.In short, they are very good for all of the above, especially for general-use TV/movies/video games. Bringing the boom is a big factor there, and the excellent soundstage translates well to video and gaming.For podcasts and pure spoken word, they are fine, but I prefer more vocal-forward sets, as I just don't have a need for the recession of vocals or a big sound stage when listening to a podcast.For gaming, general gaming is great, but if I was playing a FPS or another game where extreme clarity was needed at the high end and/or on soft sounds, I'd likely go with a set more tuned for high-end detail (I have the Beyerdynamic MMX 300).All around, love these. Solid A grade.